Abstract: At present, it has been widely accepted that fracture mechanics is the most effective approach to assessing the fatigue life of welded structures. But in the use of this method, there existed some ambiguous phenomena. Especially the initial cracks, which have significant effect on fatigue life, are set different values by different researchers when no crack could be detected under the present technology condition. That made the assessed fatigue life different widely. Focusing on these problems, when fracture mechanics is utilized to assess the fatigue reliabilities for the welded structures, analyses and clarifies how to use the stress amplitude rightly and how the initial cracks and critical cracks affect the fatigue life. Meanwhile, for the welded box girders of crane, the Monte Carlo simulation of fatigue reliabilities is performed and through the demonstration of test, the mean and standard deviation of initial cracks are researched and confirmed. The research conclusions would provide reliable reference for the assessment of the fatigue life of fracture mechanics in the future.

Key words: Fatigue reliabilities, Initial cracks, Monte Carlo method, Welded box girders of crane